Enable nodepool delete after upload option

This enables the nodepool delete-after-upload option with keep-formats
set to qcow2 on x86 image builders. This should clear out vhd and raw
files after uploads for those formats are completed keeping only qcow2
longer term. This should reduce disk space overhead while still enabling
us to convert from qcow2 to the other formats if that becomes necessary.

Note that we do not enable this for arm64 before arm64 builders
currently build raw images only and we still want at least one copy of
the image to be kept even if it is raw (and not qcow2).

Change-Id: I6cf481e0f9a5eaff35b5d961a084ae34a49ea6c6

nodepool/nb04.opendev.org.yaml

@@ -104,6 +104,9 @@ diskimages:
   - name: base
     abstract: True
     rebuild-age: 604800  # 7 days
+    # No delete after upload for arm64 images because we only
+    # build raw images for arm. In that case we don't want to delete
+    # anything because there is only one copy.
     elements:
       - block-device-efi
       - vm

nodepool/nodepool.yaml

@@ -128,6 +128,9 @@ diskimages:
   - name: base
     abstract: True
     rebuild-age: 604800  # 7 days
+    delete-after-upload: true
+    keep-formats:
+      - qcow2
     elements:
       - vm
       - simple-init